This invention relates to an antenna system for vehicles or cars, more specifically to a car antenna system suitable for transmission and reception of radio waves at frequencies of nearly 27 MHz called citizens band waves (hereinafter referred to as CB waves).
In general, this type of antenna system is so constructed that the length of an antenna element may be equivalently equal to the quarter wavelength of the CB wave, the so called loading coil being attached to the bottom, middle, or top portion of a rod antenna element set up on a car body wall. Although necessary for the CB waves, the loading coil is not requisite for the reception of amplitude-modulated waves (AM waves) for frequency-modulated waves (FM waves) transmitted from broadcasting stations. Therefore, if the loading coil is attached to any projecting part of the antenna element outside the car body, it may be seen from the outside that the car is equipped with an expensive CB-wave transmitter or a CB set, sometimes inducing mischievous acts on the loading coil or stealing of the CB set.
Although such trouble may be eliminated by locating the loading coil inside the car body, there have hitherto been proposed no satisfactory means to attain this. The reason for such situation is that a generally used quarter-wave antenna element is so designed as to attain the largest current value and the smallest voltage value or zero at the grounding point, so the internal attachment of the loading coil will shift the zero voltage point and create in the vicinity of the car body wall a portion where the voltage is not zero. As a result, the existence of capacitance caused between the antenna element and the car body wall or the ground will substantially increase the dielectric loss of the antenna element, thereby reducing the radiation efficiency of the antenna.
FIG. 1A shows an outline of the construction of the prior art antenna element attached to a car body wall. As illustrated, the bottom end portion of an antenna element 100 is coupled to a conductor 102, the bottom end portion of which is firmly fitted in an insulator barrel 104. The insulator barrel 104 is fixed by a metallic lower mounting member 106 extending obliquely upward. In attaching the antenna element 100 to a car body wall 108, the antenna element 100 is inserted in an opening bored through the car body wall 108, and fitted from above with an upper mounting member 110 formed on an insulating material. When a metallic clamping nut 112 is screwed down on the conductor 102, the upper and lower mounting members 110 and 106 are moved so as to approach each other, securely holding the body wall 108 between. Thus, the antenna element 100 is firmly attached to the body wall 108. C4 designates a capacitance formed between the body wall 108 and the conductor 102 through the insulator barrel 104, while C5 designates a capacitance formed between the body wall 108 and the conductor 102 through the upper mounting member 110. In order to illustrate the capacitances C4 and C5, the sectional marks (batches) on the relevant members are partially omitted. This also applies to the cases of FIGS. 3A and 3B as mentioned later. FIG. 1B schematically shows that the capacitances C4 and C5 are produced between the body wall 108 and the conductor 102. According to such arrangement of the capacitances, the capacitance between the body wall 108 and the conductor 102 is equal to the sum of C4 and C5.